Most modern aircraft with reciprocating engines employ a cooling system known as “pressure cooling”. Pressure cooling is accomplished by placing a cowling around an engine and then using a system of baffles and seals to induce airflow around the engine cylinders to achieve even cooling with minimum drag. Most pressure cooling systems are “down-draft” type systems where, in conjunction with the placement of the air inlet and outlet, the baffles and seals create a high pressure region above the engine and a corresponding low pressure region below the engine. The resulting pressure differential between the two regions produces a top-to-bottom airflow around the engine cylinders.
The baffles are typically of aluminum sheet metal construction and attach to brackets on the engine. The baffles extend from the engine almost to the engine cowl, and there is normally a small gap (1 to 4 inches is common) between the baffle and the cowl to allow for engine vibration and movement. This gap is sealed by baffle seals.
Baffle seals are typically made from a flexible material, such as neoprene or silicone rubber, and they are sometimes reinforced with fiberglass. The baffle seals are commonly stapled or riveted to the baffles and extend to the cowl to prevent air from by-passing the baffles. The baffle seals are typically wider than the gap they must seal, and the excess seal material bends forward such that the pressure differential between the high pressure and low pressure regions forces the baffle seal against an inner surface of the cowl (also referred to herein as “cowl contact surface”).
Due to constant flexing, mishandling during cowl installation, and a harsh operating environment, baffle seals have a limited useful life and must be replaced as part of regular maintenance. There are currently three types of baffle seal material commonly used to replace baffle seals: 1) un-reinforced silicone rubber (e.g., Federal Specification ZZ-R-765 Class 2B Grade 60 Silicone); 2) fiberglass reinforced silicone rubber (e.g., AMS 3320 Glass Cloth Reinforced Silicone Sheet); 3) neoprene coated fiberglass (e.g., AMS 3783 Chloroprene Coated Glass Cloth, a.k.a., T8071). All three of these materials are available in bulk from many aircraft supply companies. Replacement seals ordered from aircraft manufacturers appear to be either AMS 3320 Glass Cloth Reinforced Silicone Sheet or AMS 3783 Chloroprene Coated Glass Cloth, depending on the manufacturer and the aircraft vintage.
All three types of baffle seal material have a common shortcoming; they do not have a sufficiently low coefficient of friction. A low coefficient of friction is especially important, not just to extend the life of the baffle seals, but also to prevent damage to the cowl and attaching hardware. Friction between the baffle seal and the cowl contact surface transfers engine vibration to the cowl. This vibration locally erodes the cowl where the baffle seal contacts it and fatigues the cowl and all hardware attached to it, which eventually necessitates costly repairs.
Using the ASTM D 1894-01 test method with an opposing surface of stainless steel with a #8 finish, a cross head speed of 6 inches per minute, and modified with 0.25 psi surface pressure instead of 0.07 psi to more accurately reflect the conditions under which the materials are used, we found AMS 3783 Chloroprene coated fiberglass to have a static coefficient of friction of 0.616 and a kinetic coefficient of friction of 0.495. We found ZZ-R-765 Class 2b Grade 60 Silicone to have a static coefficient of friction of 2.28 and a kinetic coefficient of friction of 3.02.
Replacement baffle seals are commonly coated with a powder for shipping purposes, but this powder is quickly rubbed off, either before or during installation or when the baffle seal interacts with the cowl. This powder is not part of the baffle seals, and it offers no sustained reduction in the material's coefficient of friction. It should be understood that “baffle seal” and “sheet of material” as used herein do not include powders used topically for shipping or otherwise that do not provide more than a momentary reduction in coefficient of friction.